This cluster of human iPS cells has been induced to differentiate and express proteins found in dopamine-producing neurons. The proteins are visible with colorful fluorescent antibodies.Courtesy of Dr. Ole Isacson, McLean Hospital and Harvard Medical School.

Since the discovery of human embryonic stem cells in the late 1990s, scientists have been racing to bring stem cell therapy into the clinic. The therapy would involve transforming stem cells into specific cell types, and using them to replace tissues that have been damaged by disease. Such transplants are being put to the test in clinical trials for spinal cord injury and age-related macular degeneration.

Stem cell research is advancing on other fronts, too. It is now possible to take a sample of skin cells or other body cells, and reprogram them to behave like embryonic stem cells. These reprogrammed cells are called induced pluripotent stem (iPS) cells, and in theory, they could be derived from a patient afflicted with a disease, corrected for defects and transplanted back into the patient's body.

Scientists are also using patient-derived iPS cells to study disease mechanisms and test potential therapeutic drugs – all in a test tube. In 2009, the National Institute of Neurological Disorders and Stroke (NINDS) funded three consortia to develop iPS cell lines from individuals with Parkinson’s disease, amyotrophic lateral sclerosis (ALS), and Huntington’s disease. By coaxing these cells to become specialized types of neurons, researchers can examine how these neurons die in each disease and test drugs that may be able to slow or prevent neuronal death.

iPS Cells as Models of Disease

Embryonic stem cells are responsible for generating all of the body's cell types. During early development, they are poised in a state of indecision, ready to become muscle, bone, nerve or other cell types as needed. This is called pluripotency, and researchers have discovered that it depends on a unique genetic program. By 2008, several research labs found that by adding just a few genes into adult human skin cells, they could turn back the cells’ developmental clock and revert them to a pluripotent state, much like embryonic stem cells. Scientists quickly realized the potential of these human iPS cells to serve as models of disease.

"For neurodegenerative diseases in general, most of the existing cellular and animal models we have do not perfectly replicate the human condition," said Margaret Sutherland, Ph.D., a program director at NINDS. "We thought that iPS cells derived from patients would provide an additional tool to augment drug discovery and provide further insights into the cell biology behind neurodegenerative diseases."

"The passage of the American Recovery and Reinvestment Act (ARRA) in 2009 created an opportunity for a large, concerted effort to develop iPS cell models of neurological disease," Dr. Sutherland said. After a call for applications, each of the three iPS cell consortia was funded with a $1.9 million ARRA grant from NINDS, with additional support from the Michael J. Fox Foundation, the ALS Association, and CHDI Foundation, which supports research on Huntington's disease.

In December 2010, the consortia researchers met at a workshop near the NIH campus in Bethesda, Maryland to discuss ongoing progress and challenges. They have generated and characterized dozens of iPS cell lines from patients and healthy individuals, and have begun using the cells to probe disease mechanisms. As new iPS cell lines are generated, they are deposited into the NINDS human cell line repository at the Coriell Institute for sharing with the broader research community.

Parkinson's disease affects a part of the brain called the substantia nigra, destroying neurons there that produce the chemical dopamine. Consortium researchers have successfully generated iPS cells from individuals with the disease, and in turn have used those cells to generate dopamine-producing neurons.

Most cases of Parkinson's disease are sporadic, meaning that the cause is unknown. But there are about a dozen genes associated with the risk of developing Parkinson's, and in some individuals, the disease is caused by a mutation in just one such gene. Consortium researchers are deriving iPS cells from individuals with sporadic Parkinson's, as well as from individuals with mutations in the genes most strongly linked to Parkinson's, including alpha-synuclein, LRRK2, parkin and PINK1.

Already, an NINDS-funded team has used iPS cells to investigate the functions of PINK1.1 Previous research suggested that PINK1 maintains cellular health by disposing of old or damaged mitochondria, which are cellular energy factories. But those studies were done in animal models and non-neuronal cells. In a new study, Dimitri Krainc, M.D., Ph.D., an investigator at Massachusetts General Hospital in Boston, examined iPS-derived neurons from people with PINK1 mutations. He found that the cells accumulate abnormal mitochondria, and that this could be reversed with delivery of normal PINK1.

ALS attacks muscle-controlling nerve cells called motor neurons, leading to weakness and muscle wasting. Like Parkinson's disease, ALS is usually sporadic but about 10 percent of cases are genetic. Consortium researchers have derived iPS cells from individuals with the most common mutations that cause ALS (in the genes SOD1 and FUS) and from individuals with sporadic ALS. They have successfully converted these iPS cells into motor neurons.

The hope is that a common picture of ALS will emerge by studying iPS cells from different patients. However, not all iPS cell lines are equal. Some lines appear to have a low efficiency for generating certain cell types, including motor neurons. Researchers worry that such differences between cell lines will obscure any differences related to ALS.

Kevin Eggan, Ph.D., and colleagues at the Harvard Stem Cell Institute recently analyzed 16 iPS cell lines from ALS patients and healthy individuals.2 All 16 lines were able to become – or differentiate into – motor neurons, but with different efficiencies that depended on the unique genetic makeup of each donor. It will be important to factor in this donor-related variability as the consortia move forward, the researchers said. The good news is that the iPS cells' ability to differentiate was not affected by different lab protocols or handling in independent labs.

In a related effort, Dr. Eggan worked with Alexander Meissner, Ph.D., at Harvard to develop a "scorecard" of genetic markers that can be used to predict the differentiation capacity of new iPS cell lines.3

Huntington's Disease Consortium- Led by Leslie Thompson, Ph.D., University of California, Irvine

Huntington's disease attacks neurons in the striatum, a brain region that controls movement. The disease is caused by mutations in the huntingtin gene. In the language of DNA, these mutations consist of the repeating three-letter phrase CAG. The risk of Huntington's varies with the number of these CAG repeats, which can be counted through genetic testing. Most people have fewer than 27 repeats and are not at risk for Huntington's. Individuals with 36 or more repeats are likely to develop Huntington's. Higher repeat lengths are associated with earlier onset and more rapid disease progression.

The consortium researchers have derived iPS cell lines from individuals with and without Huntington's disease, harboring a range of 20-180 repeats. They have found that they can convert these cells into striatal neurons, and are investigating how CAG repeat length affects the health and survival of iPS-derived neurons.

Looking Ahead

In April 2011, NINDS announced the availability of two years of additional funding for the consortia, and asked the investigators to submit proposals for the continued development, study and sharing of iPS cells. This work would include expanding the number of iPS cell lines and the number of disease-related mutations represented, as well as characterizing each line's pluripotency.

Importantly, much work remains to be done to establish that patient-derived iPS cells can serve as useful models of disease and as tools for drug discovery. Investigators hope to confirm that, like the neurons living inside patients, neurons derived from patient iPS cells show evidence of distress, degeneration, or vulnerability. Once such defects are identified and carefully accounted, potential drugs could be tested for the ability to correct them. Dr. Krainc's study on PINK1-deficient neurons is one example of progress on this front. Yet, neurons are not the only cells of interest. The consortia are also looking for signs of disease in patient-derived glia. These support cells in the nervous system outnumber neurons and play influential roles in many neurological diseases.

Meanwhile, NINDS is working with the consortia to establish standard protocols, and to ensure the quality of iPS cell lines within the NINDS-Coriell repository. For example, Dr. Eggan's scorecard for measuring differentiation capacity will be used to evaluate all cell lines submitted to the repository. Dr. Sutherland said she hopes that the standards used by the consortia will make it easier for all researchers to work with iPS cells and use them in broader therapeutic efforts.

"If the iPS cell repository proves to be a sustainable, useful resource, it will give us a strong case for using the same approach to tackle other diseases," she said.